The present invention pertains to an aqueous inkjet ink comprising a particular vehicle which provides good print reliability and excellent decap performance (or latency) with dye colorants.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
A good ink-jet ink is characterized by a number of necessary properties, including color, jettability, decap time (latency), drying time and shelf-life, among others. However, there is often a tradeoff between these properties because improving one property can result in the deterioration of another property.
Ink-jet printhead technology has developed to deliver very small drop sizes. Drop volumes of 1-2 pL are currently possible, which produce extremely high resolution images with no visible graininess. In addition, smaller drops allow an image to be formed with less ink volume per unit area, thus reducing dry-time and improving printer throughput. However, in order to obtain highly colored and chromatic images with less ink, more colorant must be incorporated into the ink.
High colorant concentrations can lead to problems with print reliability and latency. Latency (also known as decap) is the amount of time a printhead can be left uncapped and idle and still fire properly, without the drying ink causing blocked nozzles, mis-directed drops or some other print defect. Problems are exacerbated by the small diameter of the nozzles designed to deliver small volume drops. The smaller nozzles can clog more easily and require a greater force to clear any high viscosity blockage.
An additional problem with higher dye concentration inks is that the dye is more likely to aggregate as it dries on the media, such as glossy photographic paper, causing a phenomenon of “bronzing” whereby specular reflected light appears colored. This is most common in cyan dyes where a reddish bronzing is seen, but can occur with other dyes. Bronzing also increases the glossiness of an the image relative to the substrate causing gloss non-uniformity between different colored areas and the white background. U.S. Patent Publication 2004/0003755 discloses inks with antibronzing properties, in particular inks comprising cyan dye.
U.S. Pat. No. 5,580,373 discloses various compounds, including butyl urea and 1,3-dimethyl urea, as anticurl agents. To be effective, it is taught that anticurl agents must be present in an amount of at least 10% by weight of the total ink. Acceptable ranges for the anti curl agents are indicated to be 10-75 wt %, preferably 12-55% and most preferably 15-30 wt %.
There is still a need for an ink-jet ink formulation that provides outstanding latency (longer decap times) while maintaining reliability and other important jetting characteristics. In particular, there is a need for such a formulation where the colorant is a dye present in relatively high concentration. Still further there is needed a formulation that will provide the foregoing properties with little or no bronzing.